Spinner angular velocity errors in laser printers have previously been minimized by two very expensive and complex techniques. One of these techniques is the use of a frictionless, air bearing suspension system for a phase lock servo controlled spinner. This is the most expensive and complex technique.
Another technique uses an output data clock for scan line data stored in an electronic memory. The output data clock may be generated by splitting the energy from the main recording laser beam so that a portion of the energy impinges on a precisely ruled high resolution grating. The energy passed by the high resolution grating is then sensed by an optical detector so as to obtain the information needed to make the correction in the motor speed. This, in turn, corrects the rate with which the data is generated. An optical encoder coupled to the spinner facet assembly will also generate an output data clock.
In both output data clock cases, a multiplying phase lock loop is required to multiply the grating or encoder output frequency up to the output data clock frequency. In addition, the grating requires auxiliary beam optics and an auxiliary beam detector which results in a more complex optical design. The optical encoder also requires a more complex spinner assembly in addition to encoder output electronic.
Since all of the laser printer designs discussed above require much additional auxiliary equipment, they are more complex and more expensive than the present invention, which utilizes only the start-of-line signal generator which is required by all laser printer designs.
More particularly, reference is made to U.S. Pat. No. 4,067,021 which issued to Howard Baylis et al. on Jan. 3, 1978 and which relates to an optical scanning apparatus comprising a source of light with which is employed a rotatable reflecting element having a plurality of reflective facets. In this apparatus, each facet of a multi-facet arrangement causes a focused beam to scan across the surface of a medium with a drive being provided for moving the recording medium perpendicular to the direction of scanning.
In this apparatus are employed an incremental type optical encoder direct coupled to the facet assembly and to the spinner drive motor and a frequency multiplying phase lock loop. This provides the output data clocking frequency for laser beam modulation. A start-of-line photocell senses the laser beam before each scan and gates the laser beam modulation clock on, thus removing facet-to-facet errors. This arrangement differs from that of the present invention for the reasons given above and as will be seen in greater detail hereinbelow.
Attention is further directed particularly to U.S. Pat. No. 4,257,053 granted to Cecil Gilbreath on Mar. 17, 1981. In this patent is disclosed a high-resolution laser graphics plotter for plotting data on a recording medium as selectively positioned spots or pixels of variable intensity. The intensity of a laser beam is modulated to produce a spot on a light-sensitive film. A multi-facet rotating mirror enables the modulated beam to scan across an image plane. This arrangement employs a flat field scan lens positioned between the rotating mirror and the image plane to provide correction for the non-linear velocity of the beam as it is scanned across the image plane.
This arrangement employs an optical encoder direct coupled to the DC motor and facet assembly and a frequency multiplying phase lock loop. This provides the output data clocking frequency for laser beam modulation. The optical encoder produces 48,000 cycles per revolution (8000 cycles for each of six scanning facets). The phase lock loop multiplying factor is 96, which results in 768,000 cycles for each of the six scanning facets. Logic circuits, which sense angular position, are used to correct facet-to-facet and facet-to-axis errors.
A micro-stepping motor assembly is provided to advance the film across the image plane. As will be seen hereinbelow, the present invention provides improvements and features which are not to be found in this patent.